The invention concerns a measuring process and an appropriate apparatus on a CNC machining center, preferably for the production of gear toothed gearing and/or other profiles amenable to generation, and for tooth finishing and supplementary machining operations.
It is well known that with the advent of CNC gear grinding machines and with it in particular of position-regulated axes and modern sensor technology, a substantial productivity increase in gear grinding has been attained, which in turn has triggered a growing demand for short down-times and a divorcement of the operator from the now rapid working cycle of these machines. Parallel to this a trend towards high quality workpieces of ever higher precision and form accuracy is to be observed. Alongside measures reducing cutting time it is therefore predominantly workpiece change and other preparatory operations at the start and finish, and between two consecutive grinding operations that were, and are being, increasingly automated. Along with the divorcement of the operator from the working cycle of the machine, however, the control of the grinding process evolving from experience is lost, and with it the opportunity to make corrections early enough to prevent machining errors. Hence the new technology of automatic high performance grinding harbors a growing risk potential for rejects or workpiece errors that entail high cost subsequent amendment.
Regarding the task of gear grinding, the differing amounts of tooth flank grinding allowance due to hardening distortion must be so removed as to obtain the ultimate flank form of micrometric dimensions. Economic considerations call for material removal in the shortest possible time. This is restricted by thermally induced crystal structure changes in the workpiece edge zone, the so-called grinding burn. The causes of this phenomenon have been largely investigated.
There are tooth flank grinding machines in existence which provide for the automation of process preparatory and/or process monitoring functions as individual operations, such as mesh aligning or centralizing, as the positioning of the tooth space relative to the generating grinding worm prior to the grinding process is termed. A typical design form of this is shown in U.S. Pat. No. 4,755,950. The characteristic of this solution on a gear tooth form grinding machine is a stationary special feeler which sinks into a tooth space and responds to contact with a tooth flank. The workpiece is subsequently rotated until the second flank makes contact. A computer registers the positions of contact and calculates the mid-position to which the workpiece is turned to centralize it.
In CH 682 853 A5 a simplification of the task is proposed by reducing the number of motions to be performed. A preferred design form of the invention makes use of a sensor with a ball gauge, which makes contact with either the tooth crest or the flanks of a tooth space of the workpiece. Thus the ball gauge registers the deviations both in the radial and in the tangential direction. The scanning system is arranged on the tool column.
At the present level of technology, the workpiece with pre-cut teeth continues to be placed on the work fixture and, at operating speed, subjected to a 100% inspection of all the right and left flanks of the gear. This consists in scanning the tooth flanks with a proximity response stationary inductive or capacitive pick-up. The sensor checks whether the correct gear is set up, and whether the tooth thickness and the radial runout are within the pre-set tolerance range. In an appropriate electronic system a pattern or model is compiled corresponding to the gear, and evaluated with respect to tooth thickness, radial runout and number of teeth. If the values are within tolerance, the rotary offset is determined on the basis of which the automatic electronic mesh aligning of workpiece and grinding worm can take place.
A common feature of the examples mentioned is the automation of the mesh aligning, which must be performed prior to grinding. For this a suitable sensor of various typexe2x80x94contacting or non-contactingxe2x80x94is arranged on the gear manufacturing machine according to special rules. These stated solutions still have considerable disadvantages, however. They are only designed for the mesh aligning task or for inspection prior to grinding. Contacting feelers require the stopping of the work spindle in order to allow their insertion in the tooth space, which prolongs the cycle time. Hence these solutions at least are not suitable for high production gear grinding, as the measurement cannot be undertaken at operating speed.
With the known layout principles, devices and processes, only the mesh aligning is realized. The recording of further geometric and technical processing characteristics during and at the end of grinding is not provided for, and therefore requires further complicated devices, which basically restrict the non-collision clearance in the working area.
Another category of devices is proposed for the registering of grinding burn. From DE 195 44 620 A1, a process for index-generating and profile grinding is known which records the change in the dielectric function in the high frequency range via the change in the capacity of an electrical resonant circuit. To this purpose the sensor must be sunk into the tooth space to within millimeter distance of the ground tooth flank. Hence, as with contacting pick-ups, this solution too requires the stoppage of the work spindle.
In DE 40 25 552 C1 it is suggested that change in the distance between the end faces of the workpiece be measured as substitute value for the temperature rise, and hence the occurrence of grinding burn, due to the grinding process. In accordance with the invention, pairs of measuring systems are fitted to grinding arbors, preferably with strain gauges as measuring element. According to the inventor, the temperature rise in grinding causes abt. 10 to 20 microns in face width change. By comparing with a reference workpiece, it is said that on workpieces subsequently ground with the same grinding data grinding burn can be reliably recognized. This method can only be contemplated for small workpieces, however, as with gears of greater mass the workpiece warming due to grinding is too little for such a measurement.
Based on the solutions prevailing, it is generally characteristic for the level of technology that for the measuring and inspection operations described there are certainly solutions available, but their application on a gear flank grinding machine causes a considerable restriction of the working area, and mostly a high expenditure in resources for equipment and measuring media, as well as long setting up and additional down-times.
The objective of the present invention, therefore, is to introduce a process and an apparatus by which in combining a number of measuring and inspection functions in one automated measuring apparatus, and the multi-use of the apparatus and the measuring instruments, the economic exploitation of the machine is increased, the quality and availability improved, and the setting up procedure simplified.
This objective is achieved by the present invention which, in one aspect, provides a process for the automatic registering and evaluation of processing and workpiece characteristics when grinding gears. Within an automatic grinding cycle with one or more sensors on the same adapter head a number of measurements are performed. The sensors necessary for the measurement are infed to a position close to the gear tip circle and, by displacing a CNC controlled spindle parallel to the workpiece axis, brought into the position required for the measurement, which is programmed in the CNC.
In another aspect the invention concerns a tooth flank grinding machine which comprises a machine base and a work spindle located for rotation in the machine base and conceived as NC axis, with a work arbor for accommodating a workpiece. A rotationally located grinding spindle is conceived as NC axis, for the attachment of a grinding wheel. A first slide is provided for producing an NC controlled relative motion between grinding wheel and workpiece parallel to the grinding spindle axis. A second slide is displaceable at a right angle to the grinding spindle axis for producing an NC controlled relative motion between grinding wheel and workpiece at a right angle to the displacement direction of the first slide. A third slide is provided for producing an NC controlled infeed motion of the grinding wheel relative to the workpiece in radial direction to the work spindle axis. An NC control unit programmably controls all NC axes. A measuring pick-up holder is radially adjustable relative to the work spindle axis and by means of an additional NC axis is displaceable parallel to the work spindle axis. To the pick-up holder at least one sensor is fitted and connected to the NC control unit.